A major impediment to the commercialization of proton exchange membrane (PEM) fuel cells is the low activity of electrocatalysts for the oxygen reduction reaction (ORR). Platinum (Pt) is considered the best cathode catalyst toward four-electron reduction of oxygen to water in acidic environments. It also shows the lowest overpotential and the highest stability. However, Pt remains an expensive metal of low abundance, and it is thus of great importance to find Pt-free alternatives for PEM fuel cells.
There has been a considerable research on: (i) ruthenium (Ru)-based chalcogenides (e.g. chevrel-phase type Mo4Ru2Se8 and amorphous MoxRuySez and RuxSey), (ii) porphyrin-based macrocyclic compounds of transition metal (e.g. cobalt phthalocyanines and iron tetramethoxyphenyl porphyrin), (iii) vacuum-deposited cobalt and iron compounds (e.g. Co—C—N and Fe—C—N) and (iv) metal carbides, nitrides and oxides (e.g. FeCx, TaOxNy, MnOx/C). However, none of the above catalysts fully meet the requirements of electrocatalysts for ORR in PEM fuel cells due to the following deficiencies: (i) low catalytic activity, (ii) poor stability, (iii) low selectivity toward four-electron reduction of oxygen to water (large amount of hydrogen peroxide >5%) and (iv) high electronic resistance. Also, they are typically synthesized through complex routes with expensive precursors.
As such, a need currently exists for a low-cost, easily manufactured carbon-based catalyst having high activity, selectivity, and stability for ORR.